The story of an 18th-century voyage to a remote island in the Pacific, in the hopes of pinning down the distance between the Earth and the sun

James Cook's measurements of the 1769 transit of Venus. (NASA)

Later today, as the planet Venus glides across the face of the sun, people all around the world will gather to watch. For 21st-century observers, the event -- known as the transit -- is a chance to see the orbits of the solar system at work as they come into a momentary alignment. Another transit will not occur until 2117.

Unless you are very young and plan to live for a very, very long time, this is it for you.

So perhaps you feel a bit of pressure to get out your solar shades and catch the historic event. But that pressure is nothing -- nothing -- compared with the pressure that was on British Captain James Cook and the crew of the HMB Endeavour when they set sail in 1768 to reach Tahiti in time to watch the 1769 transit. It was their hope that accurate measurements of the transit could finally give scientists the data they needed to answer one of the big questions in astronomy at that time: Just how far away was the Earth from the sun? If they could answer that, they could calculate the size of the solar system as a whole.

The ship set sail in August of 1768 in anticipation of the transit that would occur the following June. Their destination was Tahiti, a tiny island of about 400 square miles that had become known to Europeans only one year earlier. It was a dangerous mission: "We ... took our leave of Europe for heaven alone knows how long, perhaps for Ever," wrote Joseph Banks, a young botanist on board the ship.

At the time, astronomers knew six of the planets in orbit around the sun (Mercury, Venus, Earth, Mars, Jupiter, and Saturn) and they knew, basically, the relative distances among them -- for example, that Jupiter was five times farther than the Earth from the sun. But absolute distances -- kilometers or miles -- were as yet unknown. A century earlier, Edmund Halley (of Halley's Comet fame) had developed a trigonometric method for getting the absolute distance, a formula that was simplified by French astronomer Joseph-Nicolas Delisle. The only catch was, they needed data from a transit, and there wasn't going to be one until 1761.

The transit of 1761 came and ... went. The data collected during the transit wasn't very good, due to weather, logistics, and observers' inexperience. Thankfully, they only needed to wait eight years until another transit, and this time they would be ready. They had to be, because the next one wasn't going to happen for another 105 years -- meaning, all of the astronomers alive in 1769 would go to their graves not knowing the answer to one of the biggest questions in their field.

And so they sailed; they sailed for eight months, making landfall in April of 1769, leaving them two months to set up their observatory. Other teams were making similar preparations in Philadelphia, St. Petersburg, the Hudson Bay, and Baja, California (two solid observation points were required for Halley and Delisle's formula). Cook's team had lost six of its 94 men en route, but, the rest had made it in time. "At this time we had but very few men upon the Sick list ... the Ships compney had in general been very healthy owing in a great measure to the Sour krout," Cook wrote.

This day prov'd as favourable to our purpose as we could wish, not a Clowd was to be seen the Whole day and the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole of the passage of the Planet Venus over the Suns disk: we very distinctly saw an Atmosphere or dusky shade round the body of the Planet which very much disturbed the times of the contacts particularly the two internal ones.

But something was wrong. The different observers in Tahiti were looking at the same transit, but getting different measurements for the exact time that Venus "touched" the sun. "We differ'd from one another in observing the times of the contacts much more than could be expected," he wrote.

The problem was right there in Cook's description, that "dusky shade round the body of the Planet." That dusky shade -- the result of the haziness of Venus's atmosphere and the "black drop effect" (you know, when you pinch your fingers together in front of a bright light and they seem to connect before they actually do) -- crushed the possibility of observations precise enough to calculate the absolute distances. The same was true not just for Cook and his crew in Tahiti, but for all of the missions around the world.

But other scientific advances negated the need for transits in order to get at the age-old question. Before 2004, new scientific tools and methods such as radar and telemetry from spacecraft enabled scientists to be a bit more precise: 92,955,807.273 miles. That's a specificity that Cook and his men could have only dreamed of -- and indeed they did.

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